Cathode ray tube



Oct. 5, 1937. M. KNOLL. 2,094,606

CATHODE RAY TUBE Filed July 26, 1934 BEAM CURRENT A/VODE (HERE/VIINVENTOR 7 MAX "011'. BY

; A :ORNEY T Patented Oct. 5, 1937 UNITED STATES v assists 2,094,606CATHODE RAY TUBE Max Knoll, Berlin, Germany, assignor to TelefunkenGesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, acorporation of Germany Application July 26, 1934, Serial No. 737,022

In Germany July 31, 1933 3 Claims. (Cl. 25027.5)

The invention relates to a cathode ray tube suitable for receivingtelevision transmissions. These tubes have a control cylindersurrounding the cathode, and to which the so-called picture 5 modulationvoltage is applied, i. e., that voltage which determines the luminosityof the luminous spot on the screen of the tube.

The invention is based upon the teaching that such control cylindertogether with an oppo- 10 sitely disposed positive electrode can beconsidered an electrical lens, whose focal distance, at low inputvelocity of the electrons, lies in the direct vicinity of the controlcylinder, namely between the latter and the positive electrode. It

15 was found from tests, that in the focus produced in front of thecontrol cylinder, the cathode beam has a very small diameter,practically only a fraction of a millimeter. In accordance with theinvention, the diameter of the control cylinder 20 and its distance fromthe anode, as well as the cathode will be so dimensioned, that the anodeopening lies in the focal distance of the control cylinder. This entailsthe advantage that the electron current emitted by the cathode nearly 25completely passes through the anode opening and is intercepted by theanode shutter to a very small extent only. The anode opening can herebyhe made so small that a very small fluorescent spot is produced on thefluorescent screen of the tube, when the anode opening is reproduced onthe fluorescent screen in the known manner by means of a collector coil.

The embodiment of the invention already practically tested, will bedescribed in the following and it is schematically represented in thedrawing in which Fig. 1 shows schematically one embodiment of myinvention, and Fig. 2 shows a graph illustrative of the principle of myinvention.

40 In Fig. 1, it signifies a cathode, H is a disk which may have appliedthereto the potential of the cathode l0, or the potential of the controlcylinder l2, or any other potential therebetween. l3 designates an anodeshutter, whose opening is 45 in the axis of the control cylinder 12. Thediameter b of the control cylinder was in the mentioned experiment 10mm., the distance a of its edge opposite the anode shutter from thelatter was 2 mm., and the distance 0 between the anode 5 and cathode was6 mm. The control cylinder l2 had anegative potential of 150 voltsagainst the cathode, the anode 33 a positive potential of 2000 voltsagainst the cathode. The diameter of the emitting cathode surface :11was 4 mm., the open- 55 ing f of the anode was only 0.6 mm. Under theseconditions, it was observed that an electron current of 2 ma. passedthrough the anode opening 1, while the anode shutter 83 received acurrent of less than 0.2 ma.

This result is next insofar remarkable in that 5 it proves thatelectrons radiated by a cathode placed within a control cylinderactually pass at one place through a negligibly small cross sec-- tion,which can be designated as focal point of the arrangement. A furtherproof for the presonce of such a focal point will be noted from theresults of tests one of which is shown in Fig. 2.

In this figure there is plotted in dependence upon the distance a, therelation of the ray current to the anode current (i. e., the proportionbetween 15 the current passing through the anode opening and the currentreceived by the anode shutter). As can be seen, the proportion betweenray current and anode current has an optimum at a. definite distance ofthe control cylinder from the anode. This signifies moreover that forother values for the distance between control cylinder and anode, theanode opening is not in the focal point of the cathode ray beam, but israther more or less outside this point, and that therefore this openinggives in this case passage to only a small part of the current emittedfrom the cathode.

The result explained of Fig. 1 is also insofar remarkable in that itshows, that by suitably dimensioning of the control cylinder etc., thecurrent emitted from a comparatively large surface of the cathode canpass nearly completely through an opening I which is extremely small incomparison with the cathode. Since the specific emission of the cathode,i. e., the electron current emittedfrom a square millimeter of thecathode surface, cannot further be increased without shortening the lifeof the cathode, the above is of great importance, and also for thereason that for producing a small luminescent spot on the screen of thetube, small anode openings are of advantage.

Having thus described the invention, what I claim is:

1. In a cathode ray tube, the combination of a cathode having anelectron emitting surface, an apertured disk shield electrode concentricto said cathode and lying in the same plane as the emitting surface ofthe cathode, a cylindrical focusing electrode coaxial with andsurrounding said cathode and shield, and an accelerating annular anodecoaxial with the aforesaid electrodes and placed in register with saidcathode.

2. In a cathode ray ,tube, the combination of a cathode having anelectron emitting surface,

an annular shield electrode, said electrode lying in the same plane asthe emitting surface of the cathode, a cylindrical focusing electrode,and an annular accelerating electrode, all of said electrodes beingcoaxial with said cathode, said accelerating electrode being positionedat the focal distance of the cylindrical focusing electrode underoptimum operating conditions.

3. The combination of a cathode having an 10 electron emitting surface,an apertured disk

